RENO, Nev. — Tiny meteorites found in the Sierra foothills of Northern California likely were part of a giant fireball that exploded in daylight with about one-third the explosive force of the bomb dropped on Hiroshima at the end of World War II, scientists said Wednesday.

The rocks each weighed about 10 grams, or the weight of two nickels, said John T. Wasson, a longtime professor and expert in meteorites at UCLA's Institute of Geophysics and Planetary Physics.

Experts say the flaming meteor was probably about the size of a minivan when it entered the Earth's atmosphere with a loud boom about 8 a.m. PT Sunday. It was seen from Sacramento, Calif., to Las Vegas and parts of northern Nevada.

An event of this size might happen once a year around the world, said Don Yeomans of NASA's Near-Earth Object Program Office at the Jet Propulsion Laboratory in Pasadena, Calif. "But most of them occur over the ocean or an uninhabited area, so getting to see one is something special," Yeomans added.

"Most meteors you see in the night sky are the size of tiny stones or even grains of sand, and their trail lasts all of a second or two," he said.

The fireball was probably the size of a minivan weighing about 154,300 pounds (70 metric tons), estimated Bill Cooke, a specialist in meteors at NASA's Marshall Space Flight Center in Huntsville, Ala.

At the time of disintegration, he said it probably released energy equivalent to a 5-kiloton explosion. The nuclear bomb dropped on Hiroshima was 15 kilotons.

The boom, an expert said, was caused by the speed with which the space rock entered the atmosphere. Meteorites enter Earth's upper atmosphere at somewhere between 22,000 miles (35,000 kilometers) per hour and 44,000 miles (70,000 kilometers) per hour — faster than the speed of sound, thus creating a sonic boom.

The friction between the rock and the air is so intense that "it doesn't even burn it up, it vaporizes," said Tim Spahr, director of the Minor Planet Center at Harvard University.

Science editor Alan Boyle's blog: "Astronaut Abby" is at the controls of a social-media machine that is launching the 15-year-old from Minnesota to Kazakhstan this month for the liftoff of the International Space Station's next crew.

Wasson told the AP that one meteorite was found near the town of Coloma, about 35 miles (55 kilometers) northeast of Sacramento and another was discovered to the west near Lotus between Auburn and Placerville.

He said they were located by collectors who were knowledgeable but did not identify them.

"I'm sure more will be found, I'm hoping including some fairly big pieces," said Wasson, who has been at UCLA since 1964 when he obtained his doctorate in nuclear chemistry at the Massachusetts Institute of Technology. "The fact that two pieces already have been found means one knows where to look."

Bits of the meteor could be strewn over an area as long as 10 miles (16 kilometers), most likely stretching west from Coloma, where James W. Marshall first discovered gold in California, at Sutter's Mill in 1848.

Wasson suspected hundreds of dealers and collectors already have joined the search. He said it was important to recover the meteorites soon because any rain will cause them to degrade, losing their sodium and potassium.

"From my viewpoint as a meteorite researcher, I'm hopeful some big pieces are found right away," he said.

AP science writer Seth Borenstein contributed to this story from Washington.

Southern stargazing

Stars, galaxies and nebulas dot the skies over the European Southern Observatory's La Silla Paranal Observatory in Chile, in a picture released on Jan. 7. This image also shows three of the four movable units that feed light into the Very Large Telescope Interferometer, the world's most advanced optical instrument. Combining to form one larger telescope, they are greater than the sum of their parts: They reveal details that would otherwise be visible only through a telescope as large as the distance between them.
(Y. Beletsky / ESO)
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A balloon's view

Cameras captured the Grandville High School RoboDawgs' balloon floating through Earth's upper atmosphere during its ascent on Dec. 28, 2013. The Grandville RoboDawgs’ first winter balloon launch reached an estimated altitude of 130,000 feet, or about 25 miles, according to coaches Mike Evele and Doug Hepfer. It skyrocketed past the team’s previous 100,000-feet record set in June. The RoboDawgs started with just one robotics team in 1998, but they've grown to support more than 30 teams at public schools in Grandville, Mich.
(Kyle Moroney / AP)
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Spacemen at work

Russian cosmonauts Oleg Kotov, right, and Sergey Ryazanskiy perform maintenance on the International Space Station on Jan. 27. During the six-hour, eight-minute spacewalk, Kotov and Ryazanskiy completed the installation of a pair of high-fidelity cameras that experienced connectivity issues during a Dec. 27 spacewalk. The cosmonauts also retrieved scientific gear outside the station's Russian segment.
(NASA)
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Special delivery

The International Space Station's Canadian-built robotic arm moves toward Orbital Sciences Corp.'s Cygnus autonomous cargo craft as it approaches the station for a Jan. 12 delivery. The mountains below are the southwestern Alps.
(NASA)
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Accidental art

A piece of art? A time-lapse photo? A flickering light show? At first glance, this image looks nothing like the images we're used to seeing from the Hubble Space Telescope. But it's a genuine Hubble frame that was released on Jan. 27. Hubble's team suspects that the telescope's Fine Guidance System locked onto a bad guide star, potentially a double star or binary. This caused an error in the tracking system, resulting in a remarkable picture of brightly colored stellar streaks. The prominent red streaks are from stars in the globular cluster NGC 288.
(NASA / ESA)
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Supersonic test flight

A camera looking back over Virgin Galactic's SpaceShipTwo's fuselage shows the rocket burn with a Mojave Desert vista in the background during a test flight of the rocket plane on Jan. 10. Cameras were mounted on the exterior of SpaceShipTwo as well as its carrier airplane, WhiteKnightTwo, to monitor the rocket engine's performance. The test was aimed at setting the stage for honest-to-goodness flights into outer space later this year, and eventual commercial space tours.

Red lagoon

The VLT Survey Telescope at the European Southern Observatory's Paranal Observatory in Chile captured this richly detailed new image of the Lagoon Nebula, released on Jan. 22. This giant cloud of gas and dust is creating intensely bright young stars, and is home to young stellar clusters. This image is a tiny part of just one of 11 public surveys of the sky now in progress using ESO telescopes.
(ESO/VPHAS team)
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Fire on the mountain

This image provided by NASA shows a satellite view of smoke from the Colby Fire, taken by the Multi-angle Imaging SpectroRadiometer aboard NASA's Terra spacecraft as it passed over Southern California on Jan. 16. The fire burned more than 1,863 acres and forced the evacuation of 3,700 people.
(NASA via AP)
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Where stars are born

An image captured by NASA's Spitzer Space Telescope shows the Orion Nebula, an immense stellar nursery some 1,500 light-years away. This false-color infrared view, released on Jan. 15, spans about 40 light-years across the region. The brightest portion of the nebula is centered on Orion's young, massive, hot stars, known as the Trapezium Cluster. But Spitzer also can detect stars still in the process of formation, seen here in red hues.
(NASA / JPL-Caltech)
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A long, long time ago...

This long-exposure picture from the Hubble Space Telescope, released Jan. 8, is the deepest image ever made of any cluster of galaxies. The cluster known as Abell 2744 appears in the foreground. It contains several hundred galaxies as they looked 3.5 billion years ago. Abell 2744 acts as a gravitational lens to warp space, brightening and magnifying images of nearly 3,000 distant background galaxies. The more distant galaxies appear as they did more than 12 billion years ago, not long after the Big Bang.
(NASA / NASA via AFP - Getty Images)
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Frosty halo

Sun dogs are bright spots that appear in the sky around the sun when light is refracted through ice crystals in the atmosphere. These sun dogs appeared on Jan. 5 amid brutally cold temperatures along Highway 83, north of Bismarck, N.D. The temperature was about 22 degrees below zero Fahrenheit, with a 50-below-zero wind chill.